1. Field of the Invention
The present invention relates to an optical recording medium capable of retaining the secrecy of stored information and to a method of erasing, recording, and reading the stored information while retaining a high level of secrecy.
2. Discussion of the Background
Optical recording media are disc-shaped media capable of being read and recorded to by optical devices. For example, Compact Discs (hereinafter referred to as CDs) such as a Compact Disc Digital Audio (CD-DA), a Compact Disc Read Only Memory (CD-ROM), a Compact Disc Recordable (CD-R), and a Compact Disc ReWritable (CD-RW), or large capacity optical recording media such as a Digital Video Disc Read Only Memory (DVD-ROM), a Digital Video Disc Recordable (DVD-R), and a Digital Video Disc ReWritable (DVD-RW) are well known. These media have become wide spread as information recording and transmitting media. In particular, CD-RW media have become remarkably wide spread as erasable/rewrite-able CD media.
When these optical media are used as information-transmitting media, it is often important to keep information stored within the media secret. For example, data stored in a CD can be easily read by a commercially available CD drive or CD player. Therefore, a problem which occurs is that secret information is easily leaked when an optical recording medium used to store the secret information is read by an unauthorized person.
The CD-RW media can be reused by erasing information which has been stored. Information erasing methods for CD-RW include a full-erase method in which all the recorded data is erased and a quick-erase method in which only control data is erased. Control data is necessary to allow a CD drive to access the information stored in the data area of a CD-RW. The quick-erase method is used in order to quickly make a CD medium ready for rewriting.
FIGS. 4A and 4B are schematic diagrams illustrating area-structures of a CD-RW medium before and after information is erased by a quick-erase method, respectively. A prior art erase method is described hereinafter referring to FIGS. 4A and 4B.
In FIG. 4A, reference numerals 41, 42, 43, and 44 designate a program memory area 41 (hereinafter referred to as a PMA), a table of contents area 42 (hereinafter referred to as a TOC area), a program area 43, and a lead-out area 44, respectively.
User information is stored in the program area 43 of the CD-RW medium shown in FIG. 4A and a CD-RW drive gains access to the information in the program area 43 on the basis of control data recorded in the PMA 41. After the control data is additionally recorded in the TOC area 42 and the lead-out area 44, the information recorded in the CD-RW medium becomes capable of being read by a CD drive such as a CD-ROM drive. This recording operation is performed using a CD-RW drive.
When the described quick-erase method is used, all of the control data stored in the filing information areas, i.e., the PMA 41, the TOC area 42, and the lead-out area 44, are erased. The user information recorded in the program area 43 remain intact. However, there are no CD drives available which can read the user information stored in the program area 43, because there is no control data in the filing information areas.
Accordingly, there are currently no methods to read user information stored in the program area 43 of a CD-RW medium when the medium is accidentally erased by a quick-erase method.